Chem prac help!

[Rosie]

I am doing an extended experimental investigation on four pracs for my first sac and we are nearly finished.
Can you just clarify these questions for me

What distinguishes gravimetric analysis from other forms of analysis?

Explain the chemical principle involved in spectroscopic techniques in general and specifically in colorimetry?

In this assessment task, you will analyse the iron content o fthe fertiliser using a redox titration. Describe another way an anlysis for iron could be performed?

Phosphorus from fertilisers has been implicated in a type of pollution called eutrophication. Explain how this process arises.

These are background questions for my sac and if these could be answered I'll be very happy.
Thanks   

[ed_saifa]

Gravimetric analysis finds the quantity of the substance in a sample by precipitating it.

[Toothpaste]

Gravimetric analysis involves precipitation and weighing.

Spectroscopic techniques involve absorption or emission of part of the electromagnetic spectrum. In colorimetry, a light wave of certain wavelength and intensity is shone on a solution and a detector at the other end measures how much of that light goes through. Thus we can determine how much is absorbed by our solution. It uses wavelengths from the visible light region of the spectrum.

There are a lot of ways. You can use AAS.
Using an iron cathode lamp. The sample is made into a solution. It is sprayed into a flame; where the sample is atomised and the atoms absorb light of the appropriate frequency. The light from the lamp passes through it and through a monochromator. A particular wavelength is selected and detected.

Eutrophication is the increase of chemical nutrients (in this case phosphorus) in an ecosystem (i.e. water bodies).
Example:
1. Farmers use fertilisers
2. Rain washes it into a lake
3. Algae blooms occurs
(Cyanobacteria (aka blue green algae) can produce toxins - making the water source unuseable and dangerous. It can also accumulate on the surface of the lake and block sunlight from reaching aquatic plant life; hence killing them because they can't carry out photosynthesis.)

[Mao]

on spectroscopy (question 2 and 3), sift through this thread
http://vcenotes.com/forum/index.php/topic,2087.15.html

there are a lot of well explained answers that could help a better understanding =)

[Toothpaste]

Okay I have a question too.

Question:
Sulfuric acid is a catalyst in the reaction of salicyclic acid with acetic anhydride. How might a chemist prove that sulfuric acid acts as a catalyst, rather than as a reactant, in this reaction?

Answer?
If the salicyclic acid is reacted with acetic anhydride without a catalyst the process will be slower but the reaction will still happen. With the catalyst added the rate of reaction will be significantly faster since catalysts lower the activation energy needed for a reaction to occur.

I'm not answering the question properly (or even directly), what's a better way to word it?

Thanks.

This is a question on a practice sac from the heinemann workbook.

[Collin Li]

Your answer hasn't addressed the fact that a catalyst is not consumed in the overall reaction. The question says, prove sulfuric acid acts as a catalyst, rather than as a reactant.

[Collin Li]

To prove this, the most obvious answer is to add a fixed and known amount of sulfuric acid to the beaker with the reactants, and when the reaction reaches completion, measure the amount of sulfuric acid at the end. There should be no change at all. That is easier said than done, as you've got three soluble things in a solution, so you might have to suggest some better ideas.

Inconclusive evidence that would support the claim would be obtained by adding sulfuric acid in minor quantities compared to the reactants. This does not rule out sulfuric acid from being a reactant though, it could be a naturally high yield reaction, or a very strange stoichometric ratio between sulfuric acid and the other reactants and products. However, following on from this test, you could then show that adding more sulfuric acid does not increase the yield of the reaction as it normally would, if it were a reactant. Hence, sulfuric acid is acting as a catalyst and not a reactant.

N.B.: Equilibrium was struck off Unit 3, but it gave stronger insights to the workings of a catalyst. A catalyst speeds up the rate of reaction, but does not affect the yield of the reaction. It also gave a clearer insight into why adding more reactant would increase the yield of the reaction. For this reason, I don't believe this is a question you could expect on the exam. This is further supported by the fact that questions about catalysts on the old course were very straightforward and simple, unlike this question, yet you learn less about catalysts in the new course.

[Rosie]

Thanks guys for all your help
Good answers

I wanted to ask how you would know which states to put on a compound when a reaction takes place. Are there only 4 types, (g) = gas, (l) = liquid, (aq) = aqueous solutions and (s) = solid.

When do I apply these for compounds because it is always important to put these states on after a reaction, especially in the exam. 

[Collin Li]

I wanted to ask how you would know which states to put on a compound when a reaction takes place. Are there only 4 types, (g) = gas, (l) = liquid, (aq) = aqueous solutions and (s) = solid.

There are more than 4 types. The other ones that I'm aware of are when the compound is dissolved in some solvent other than water, i.e: (org) = organic solvent, or (cry) = cryolite solvent. You probably won't encounter these in VCE.

Usually the question will give you enough information, plus a bit of commonsense is required. For example, in a combustion reaction (highly energy releasing), you wouldn't expect your H2O to be a liquid.

[Mao]

and, for organic compounds

for n=1 to 4, they are gas at room temperature

for n=5 to ~10, they are liquid (petrol is octane)

for n>10, they are usually solids

alkene's boiling point < alkane, this is because there are less electrons on its outer-surface (less hydrogens), hence lower dispersion.

branched alkanes < straight alkane, this is because straight-chains pack better, so more dispersion

functional groups > alkane, functional groups are usually polar, and dipole-dipole attraction is stronger than dispersion.

[Rosie]

Prac
Regarding the synthesis of aspirin, why does aspirin act as a catalyst, rather than a reactant?

I also do not understand the percentage yield. Referring, to its equation, what does theoretical yield and actual yield mean?

[ed_saifa]

Theoretical yield means the value you get from a stoichiometric calculation whereas the actual yield is the amount you get from doing it industrially.

[Toothpaste]

Prac
Regarding the synthesis of aspirin, why does aspirin act as a catalyst, rather than a reactant?

Did you mean sulfuric acid? (aspirin is the product)

[cara.mel]

I wanted to ask how you would know which states to put on a compound when a reaction takes place. Are there only 4 types, (g) = gas, (l) = liquid, (aq) = aqueous solutions and (s) = solid.

There are more than 4 types. The other ones that I'm aware of are when the compound is dissolved in some solvent other than water, i.e: (org) = organic solvent, or (cry) = cryolite solvent. You probably won't encounter these in VCE.

Usually the question will give you enough information, plus a bit of commonsense is required. For example, in a combustion reaction (highly energy releasing), you wouldn't expect your H2O to be a liquid.

Cryolite was in it last year unit 4 in the hall herolt cell or whatever, something to do with seperating aluminium and the state was (in cryolite) I was told to write vs l, aq etc.

[Pandemonium]

Inconclusive evidence that would support the claim would be obtained by adding sulfuric acid in minor quantities compared to the reactants. This does not rule out sulfuric acid from being a reactant though, it could be a naturally high yield reaction, or a very strange stoichometric ratio between sulfuric acid and the other reactants and products. However, following on from this test, you could then show that adding more sulfuric acid does not increase the yield of the reaction as it normally would, if it were a reactant. Hence, sulfuric acid is acting as a catalyst and not a reactant.

Hmm, there's still some doubt here. For example, what if all of the reactants (save sulfuric acid) had reacted after 'adding minor quantities compared to the reactants'. Adding more sulfuric acid isn't conclusive evidence that sulfuric acid is merely a catalyst.

Of course, we must reconsider the action of a catalyst (being that it merely speeds the process and does not get consumed itself). Therefore, to test this, I think there's another method that is more useful and conclusive.

Take three pipettes (of equal and accurate volumes) from the salicyclic acid with acetic anhydride mixture and place them into three different beakers. In two beakers, drop a few drops of sulfuric acid. In the other beaker, drop as much as you want (be careful of its acidity, you don't want to get harmed). Assuming the whole reaction should be finished in 10 minutes (for the mixture that had ample sulfuric acid added), check the yield for the mixture that had ample sulfuric acid added, and one of the mixtures that had 3-4 drops after 5 minutes elapses. Leave the third beaker with the 3-4 drops for another 30 minutes or so. Check the yield.

The comparison between the mixture with the ample sulfuric acid should be of higher yield of aspirin than the one with little sulfuric acid that was measured in the same timeframe. This merely speculates that the higher yield is due to the higher concentration of sulfuric acid and the only doubtless reasoning to come out of this is that sulfuric acid plays a part in the reaction (i.e. it doesn't determine whether or not the sulfuric acid is directly a reactant.)

Compare the yields latter mixture with few drops of sulfuric acid with the prior yields found. Because we added more mols of sulfuric acid in the 'ample' mixture, if sulfuric acid were a part of the reaction (reactant) we'd expect that this result should (as opposed to would) be much higher than the two other results. However, if the latter '3-4' dropped mixture yields higher than the 'ample' mixture, we can say that due to the fact that a higher yield was obtained from the latter '3-4 dropped mixture', addition of more sulfuric acid has no bearing on the amount yielded in total and thus sulfuric acid has no bearing on the amount produced. This also means that sulfuric acid is not consumed in the process. But, because we acknowledge that addition of more sulfuric acid increased the rate of reaction, sulfuric acid must be a catalyst.

However, if the last yield doesn't exceed the 'ample' mixture, then nothing can be determined.

The reason why I designed the experiment to prematurely take out the two samples at the start is to prevent reaction to completion. If we reacted it to completion and compared to the latter '3-4 dropped' mixture, then the latter '3-4 dropped' mixture may not exceed the 'ample' mixture and thus it cannot be determined whether sulfuric acid is consumed in the reaction. If you think there is still doubt, go for a fourth beaker that has the same amount of 'ample' drops and is allowed to react for the full 30 minutes.

ALTERNATIVELY, you could say 'do the experiment without the sulfuric acid'. But I wouldn't. Two reasons:
a) sulfuric acid might be required to decrease the activation energy, without it, the reaction might not even take place.
b) might take too long for any noticeable/comparable yield to yield.